The operation of a traditional pipe cutter used by a plumber is well understood. As the cutter is rotated around the exterior of the pipe, the pipe is progressively grooved by the force of the cutting wheel and the groove deepens with each successive passage of the cutting wheel until the pipe breaks.
A close examination of the above "cutting" process will show that the pipe really is not "cut", but a groove is formed in the pipe by swaging the metal of the pipe at the "cut", and this metal undergoes cold flow as the cutter wheel wedges the metal at the groove apart. The cold flow of the metal in the pipe continues as the cutter forces its way inwardly until the separation force produced by the wedging action of the cutter wheel is sufficient to fracture the portion of the pipe wall remaining at the "cut". If the cutter wheel is operating correctly, there will be practically no swarf produced by the pipe cutter during the "cutting" operation.
The cutting tool of this application functions in a somewhat similar manner but its application is typically found in severing large cylindrical objects which have relatively large diameters and relatively thin walls and which must be "cut" in environments where (in this instance) the cylinder is immersed under several feet of water. The cutting operation for these objects thus progresses (underwater) from the inside surface of the cylindrical wall toward the exterior surface of the wall until the cylinder fractures. This invention may be advantageously applied to applications in the nuclear reactor art where reconfiguring and/or decommissioning of nuclear reactors must be undertaken. The invention may be specifically applied to good advantage in severing such items as pressure vessels or stainless steel shrouds in Boiling Water Reactors (BWR's) which typically have diameters in the order of 12-20 feet and which have a wall thickness from about one to eight inches. The ratio of the cylinder diameter to wall thickness gives some indication of the stability of the cylindrical wall (the higher the number, the greater the flexibility of the wall). For cylinders applicable to this invention, the diameter to thickness ratio may be in the order of 25-150; for a commercial steel pipe used in the plumbing industry the ratio may be as small as 4. It will therefore be apparent that the cylindrical objects to which this invention applies typically have walls which are quite flexible and which may be easily deformed by the application of localized radial force.
In reconfiguring and/or decommissioning BWR's, it is necessary to cut large cylindrical reactor components into pieces in order to make the removal of the component possible as well as to facilitate handling and storage of the cut pieces which may yet be radioactive.
Attempts to cut such components (successfully or otherwise) while the cutting equipment is immersed in substantial depths of water, have included plasma arc torch, Electric Discharge Machining (EDM), Laser cutting devices, water jets and metal cutting saws.
Most of the above devices produce significant amounts of swarf during a cutting operation. In some instances, the swarf is produced as a vapour and in other processes, the swarf consists mainly of small metallic particles. It is easily seen that the production of substantial amounts of radioactive swarf formed during the cutting process, can lead to a very serious and expensive cleanup and containment operation which must be carried out under a substantial height of water.
Other severing techniques, which have been tried with varying degrees of success, appear below.